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Sunburst (This enters the battlefield with a charge counter on it for each color of mana spent to cast it.)
Remove a charge counter from Pentad Prism: Add one mana of any color.
Once Pentad Prism has run out of charge counters, it remains on the battlefield.
Sunburst checks what mana was actually spent to cast the spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.
